See below for Selected Publications.

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Research

Our interests in basic nuclear science span from the structure of nuclei far off stability to the mechanisms by which heavy nuclei react and highly excited nuclei decay. Topics under current investigation include:

• The influence of phase transitions in finite, two component quantal systems on the dynamics of collisions between heavy nuclei. Developing techniques to measure the density-of-states for nuclei along the rapid n-capture nucleosynthesis path. Clustering in low density nuclear systems.
• The evaporation of complex clusters of nucleons and the influence of final state interactions on the decay of these clusters. Development of new detector technology and pulse processing electronic for ionizing radiation.

In addition to these basic science topics, we are also engaged in an effort with the WU department of radiology to improve the dosimeters used for brachytherapy.

Here we will discuss only the first topic in the list above. Nuclear systems are two component (n and p) quantal systems. When taken through a phase transition, quantal systems must obey the same Gibbs' conditions (equality of the chemical potentials of each substance in the phases in equilibrium) as non-quantal systems. The common component fractionation (distillation) with phase separation can also occur in multicomponent quantal systems. It is just such a component fractionation (different n/p ratios in the low and high-density regions of a reaction system), driven solely by quantal effects, which has drawn our interest for the last few years. The thermodynamic force driving such a fractionation can be understood as follows. Imagine an unequal filling of dual sets of quantum levels, one set for n's the other for p's. As a result of the different Fermi levels, there is a finite thermodynamic potential difference between n's and p's. Given enough time the Weak interaction will convert n's to p's (or visa-versa) to equalize the Fermi energies. This is in fact what drives -decay. However due to the weakness of the interaction, such interconversions have characteristic times exceeding 1 ms.

On a shorter time scale, fractionation should occur if two "phases" of different density are present. The phase with lower density will have the quantum levels spaced closer together and thus a particle imbalance will result in a smaller absolute chemical potential difference. If the two phases of different density are in equilibrium, the nucleon species in excess will be driven into the low-density phase (where the absolute difference in the Fermi levels is smaller.) Both theoretical modeling and experimentation, related to generating conditions under which such a fractionation could occur, are active research areas.

Needless to say, this topic is closely related to the Equation of State (EoS) of asymmetric nuclear matter. Decoding this EoS is essential for determining the structure of neutron stars and the nature of super-nova explosions. These stellar explosions are the likely mechanisms for the synthesis of about half of the heavy elements via the rapid neutron capture process. In this process, the explosion generates an intense pulse of neutrons which are sequentially captured by seed nuclei. After the neutron pulse subsides, the very neutron rich species -decay back to stability. This element building process is controlled by the masses and the density of states (at the energy corresponding to the capture on a neutron) of the -unstable, neutron rich nuclei.

Selected Publications

• "The Caloric Curve for Mononuclear Configurations," L.G. Sobotka, R.J. Charity, J. Töke, and W.U. Schröder, Phys. Rev. Lett., 93, 132702 (2004).
• "Continuum Corrections to the Level Density and its Dependence on Excitation Energy, n-p Asymmetry, and Deformation," R.J. Charity and L.G. Sobotka, Phys. Rev. C 71, 024310 (2004).
• "Pulse-shape Discrimination of La-Halide Scintillators," C. Hoel, L.G. Sobotka, K.S. Shah, and J. Glodo, Nucl.Instru. Meth. A 540, 205 (2005).
• "The Entropy and Caloric Curve for Mononuclei Considering both Surface Diffusness and Self-Similar Expansion Degree of Freedom," L.G. Sobotka and R.J. Charity, Phys. Rev. C 73, 014609 (2006).
• "Asymmetry dependence of proton correlations," R.J. Charity, L.G. Sobotka, and W. H. Dickhoff, Phys Rev. Lett. 97, 162503 (2006).

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